The widespread implementation of next generation ultra-large scale photovoltaic technologies (beyond 100 gigawatt peak (GWp)) will require drastically reducing production costs and achieving high efficiency devices using abundant, environmentally friendly materials. Thin-film chalcogenide-based solar cells provide a promising pathway to cost parity between photovoltaic and conventional energy sources. Currently, only Cu(In,Ga)(S,Se)2 and CdTe technologies have reached commercial production and offer over 10 percent power conversion efficiency. These technologies generally employ (i) indium (In) and tellurium (Te), which are relatively rare elements in the earth's crust, or (ii) cadmium (Cd), which is a highly toxic heavy metal.
Copper-zinc-tin-chalcogenide kesterites, with the ideal formula Cu2ZnSn(S,Se)4 (CZTSSe), more generally expressed as Cu2−xZn1+ySn(S1−zSez)4+q, wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; −1≦q≦1, have been investigated as potential alternatives because they are based on readily available and lower cost elements. However, photovoltaic cells with kesterites, even when produced using high cost vacuum-based methods, had until recently achieved at best only 6.7 percent power conversion efficiencies, see H. Katagiri et al., “Development of CZTS-based thin film solar cells,” Thin Solid Films 517, 2455-2460 (2009).
U.S. Patent Application Publication No. 2011/0094557 A1 filed by Mitzi et al., entitled “Method of Forming Semiconductor Film and Photovoltaic Device Including the Film,” (hereinafter “U.S. Patent Application Publication No. 2011/0094557 A1”) and T. Todorov et al., “High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber,” Adv. Mater. 22, E156-E159 (2010), describe a hydrazine-based approach for depositing homogeneous chalcogenide layers from mixed slurries containing both dissolved and solid metal chalcogenide species dispersed in systems that do not require organic binders. Upon anneal, the particle-based precursors readily react with the solution component and form large-grained films with good electrical characteristics and device power conversion efficiencies as high as 10%.
However, favorable electronic properties of these materials are found in a relatively narrow compositional range, i.e., Cu/(Zn+Sn)=0.7-0.9 and Zn/Sn=1-1.3. See, for example, H. Katagiri et al., “Development of CZTS-based thin film solar cells,” Thin Solid Films, 517, 2455-2460 (2009).
A common challenge found in kesterite layer fabrication is the volatile nature of film constituents at high temperature, such as tin (Sn) chalcogenide compounds. See, for example, D. B. Mitzi et al., “The path towards a high-performance solution-processed kesterite solar cell,” Solar Energy Materials & Solar Cells, 95, 1421-1436 (2011). This property makes it particularly difficult to fabricate films with desirable composition and large-grained structure at elevated temperatures.
In addition to Sn compounds, chalcogenides (sulfur (S) and selenium (Se)) are volatile at relatively low temperatures. Their beneficial effect on film crystallization has been known in CIGS films. See U.S. Patent Application Publication No. 2007/0092648 A1, filed by Duren et al., entitled “Chalcogenide Solar Cells.” This approach of extra chalcogen in the film can be extended to CZTS and has been applied in the teachings of U.S. Patent Application Publication No. 2011/0094557 A1 and U.S. Patent Application Publication No. 2011/0097496 A1, filed by Mitzi et al., entitled “Aqueous-Based Method of Forming Semiconductor Film and Photovoltaic Device Including the Film” (hereinafter “U.S. Patent Application Publication No. 2011/0097496 A1”) for all of the 5 deposited layers. Yet excess of chalcogen in the bulk of the film may lead to the occurrence of voids and cracks.
There are reports employing an anneal atmosphere containing tin-sulfur/selenium vapor in a sealed glass ampoule, including a device efficiency of 5.4%. See, for example, A. Redinger et al., “The Consequences of Kesterite Equilibria for Efficient Solar Cells,” J. Am. Chem. Soc., 133 (10), pp 3320-3323 (2011). However, precise process control in this configuration may not be straightforward in large-area applications.
Thus, improved techniques for the fabrication of kesterite layers would be desirable. In particular, improved techniques for controlling the concentration and gradient of volatile elements Sn, S and Se within the bulk CZTSSe film are required in order to target improved device performance.